Well, WD-40 is basically kerosene, which is a good solvent, but it also contains some non-volatile, waxy material, which is left behind after the kerosene evaporates. *That* is what causes problems, because it will gum up the works, and it's devilishly hard to get rid of.

WD-40: good for displacing water and removing/preventing rust, on stuff that's not camera equipment!